Abrupt changes in the impedance of an RF line result in reflections in the RF signal. In mobile communication appliances, impedance matching circuits match the antenna impedance to the impedance of the signal lines. If, by way of example, the antenna of a mobile communication appliance has a different impedance than the impedance of the signal line, the signal transmission from and to the antenna is subject to interference. In modern mobile communication appliances, antennas may be connected which have an impedance that is highly dependent on their physical surroundings. Changes in the surroundings of the antenna bring about a change in the antenna impedance. An impedance matching circuit which matches the variable antenna impedance to the impedance of subsequent stages of the mobile communication appliance is therefore frequently unavoidable.
An adaptive antenna matching network for impedance matching is known from the Patent Application US 2007/0194859 A1, for example. An impedance matching circuit which is connected between an antenna and the front-end module of a mobile communication appliance comprises adjustable-capacitance capacitive elements connected in the signal path and also inductive elements. In a “closed control loop” mode, good impedance matching is achieved in a narrow frequency range. In an “open control loop” mode, broadband matching is obtained, but with worsened insertion loss.
One problem of known impedance matching circuits is that the impedance matching generally relates to a single frequency band. In this frequency band, the impedance has good matching, but in other frequency bands it is possible neither to send nor to receive RF signals.